The present invention concerns a method of measuring, the wear, when stopped or running, on at least one tire mounted on a wheel, a tire and a wheel provided separately for implementing this method, and a tire/wheel assembly for a motor vehicle comprising the tire and the wheel. The invention also concerns methods of manufacturing the tire and a motor vehicle comprising at least one such tire/wheel assembly.
It is known how to provide wear detectors on the tread pattern elements of a tire tread for motor vehicles.
The British patent publications GB-A-2 330 808 and GB-A-2 268 715 disclose visual wear detectors consisting of layers of colored rubber reflecting light, which are disposed over the entire circumference of the tire, in the mass of the tread, and which are visible following a given amount of wear on the tire.
A major drawback of these detectors lies in the precarious nature of the wear information obtained, which can only be noted visually and possibly much after the critical wear threshold of the tire has been exceeded.
One aim of the present invention is to propose a method of measuring the wear on at least one tire comprising tread pattern elements and mounted on a wheel, which makes it possible to remedy this drawback.
To this end, the wear measuring method according to the invention comprises the step of assigning a wear variable representing the wear on the element or elements to an energy variable representing the energy of electromagnetic radiation transmitted from the external space to the internal space of the or each tire through at least one of the elements.
Tread pattern element, in the present description, means any element in relief which is intended to be in contact at one time or another with the ground, i.e. as soon as running starts, or after wear on this element has begun. This element can thus consist of a block, for example substantially parallelepipedal or cylindrical in shape, or a circumferential rib with a variable transverse section (i.e. extending over all or part of the circumference of the tread).
It should be noted that the tread pattern element according to the invention can advantageously consist of a “wear indicator,” for example in the form of a block or rib with a height substantially less than that of the tread patterns intended to be in contact with the ground during ground contact.
According to another characteristic of the invention, the energy variable itself represents the fact that an electromagnetic radiation transmission means is flush with the top surface of the top of the element, the top surface of the tread pattern elements being intended to be in contact with the ground right from the start or at a subsequent time during running.
According to another characteristic of the invention, the method comprises assigning, to a value of the energy variable less than at least one predetermined energy threshold, of a value of the wear variable corresponding to wear on the element which does not extend as far as at least one predetermined wear threshold.
According to one embodiment of the invention, the method comprises the allocation, to a value of the energy variable equal to or greater than the predetermined energy threshold, of a value of the wear variable corresponding to a wear on the element which has reached or exceeded the predetermined wear threshold.
According to another embodiment of the invention, the method comprises the allocation, to a plurality of values of the energy variable reaching or exceeding the predetermined energy threshold, of the same number of values of the wear variable corresponding respectively to levels of wear on the element reaching or exceeding the predetermined wear threshold, so that the increase in wear on the element is measured continuously between the wear threshold and a maximum wear level.
Advantageously, the electromagnetic radiation transmitted from the external space to the internal space consists of visible light, the tire or tires equipping the same motor vehicle.
Another aim of the present invention is to propose a tire for implementing this wear measurement method, the tire being delimited radially by an internal face and by tread pattern elements intended to be in contact with the ground by means of their respective tops from the very beginning or at a given moment in the running.
According to the invention, the tire comprises in its mass at least one electromagnetic radiation transmission means which is designed to be able to transmit, through at least one of the elements, incident radiation issuing from the space external to the tire to the space internal thereto, as from the time when the transmission means is flush with the surface of the top of the element.
The or each transmission means is preferably permeable to visible light (i.e. being translucent or transparent) and can be formed from one or more mineral or organic materials.
Tests carried out show that the rigidity and the area of the axial section (i.e. in the axial plane of the tire perpendicular to its median circumferential plane) of the transmission means determine the ability of the latter to remain in place during running in the course of the life of the tire. This is because a transmission means will have all the more tendency to be ejected from the tire when any local discontinuities of rigidity with the adjacent compositions of the tire are higher (i.e. this means will be more rigid), for the same area of the axial section characterizing it.
According to a first example embodiment of the invention, the or each transmission means consist of one or more relatively rigid materials each having a modulus of elasticity MA10 (at 10% deformation), measured in accordance with ASTM D 412 of 1998, which is between 1 GPa and 10 GPa (this material advantageously being based on glass, quartz or a plastic comprising a choice of a thermoplastic polymer such as a polystyrene, a methyl polymethacrylate, a polycarbonate, a polyamide, a polyvinyl chloride, a polyester or a thermosetting polymer).
It is possible for example to use one or more optical fibers opening out in the internal space, which are each formed by a core based on glass, silica or quartz and a sheath based on a thermoplastic polymer or a glass, silica or quartz with a refractive index less than that of the core.
It is also possible to use one or more optical fibers which are each formed from plastics such as those mentioned (the sheath of the optical fiber or fibers being formed from a plastic with a refractive index less than that of the core).
It is also possible to use as transmission means one or more assemblies, rectilinear or not, of optical fibers with a very small cross-section (diameter between 50 μm and 100 μm) which are each embedded in a sheath consisting of a rubber composition designed to mechanically decouple each optical fiber assembly from the adjacent rubber compositions in the tire.
Each optical fiber assembly is advantageously of the same type as those used for producing the cables used as metallic or textile reinforcements in tires (i.e. twisted cables or cables with layers with a finite or infinite winding pitch), and each assembly has an increased resistance to breakage caused by the deformations in running on the crown of the tire. It is possible for example to use assemblies or bundles of fibers each comprising from around ten to around a hundred optical fibers.
It should be noted that these optical fiber assemblies are particularly advantageous for implementing the wear measurement method according to the invention, because on the one hand of the excellent light transmission coefficient of this type of material, which makes it possible to use assemblies with a diameter of less than 1 mm, and on the other hand the absence of changes in these assemblies over time and under the thermomechanical stresses caused during the life of the tire.
Preferably, the or each transmission means consisting of this relatively rigid material or materials will have an axial section with a very much reduced area (for example like that of an optical fiber), so as to minimize any discontinuities in rigidity with the adjacent compositions and thus remain durably in place, following prolonged running.
It should be noted that this minimization of the area of the axial section of the or each transmission means can be compensated for by the use of a very high radiation transmission coefficient for this transmission means and/or by providing a plurality of such transmission means in the tire, for a given quantity of energy transmitted.
According to a second preferential example embodiment of the invention, the or each transmission means consists of at least one rubber composition based on at least one elastomer having a modulus of elasticity MA10 (at 10% deformation), measured in accordance with ASTM D 412 of 1998, which is between 1 MPa and 20 MPa (this elastomer advantageously belonging to the family consisting of cross-linkable elastomers, thermoplastic elastomers, true polyurethanes and polyurethane derivatives, such as polyurethane/urea, polyureas, polyurea/urethane, polyurethane/isocyanurate, polyurea/isocyanurate or polyurethane/urea/isocyanurate).
Preferably, use is made of dienic elastomers which can either be vulcanized or cross-linked with peroxides. It is possible to cite for example polybutadienes, polyisoprenes or styrene/butadiene copolymers. It is also possible to use elastomers with a reduced number of double bonds such as EPDMs (ethylene, propylene and diene terpolymers) or butyl rubbers (isoprene and isobutylene copolymers), halogenated or not.
Even more preferentially, use is made of polyisoprenes, styrene/butadiene copolymers, isoprene/styrene copolymers or polybutadienes.
By way of translucent rubber composition which can be used in these transmission means consisting of rubber, use is advantageously made of compositions with no staining element and strong ingredients which are not soluble or dissolve only with difficulty in the translucent composition, which comprise:                paraffin oils by way of plasticizing oil, instead of the usual aromatic or even naphthenic oils,        non-staining phenolic antioxidants as an antioxidant, instead of the usual antiozonants which result in significant browning of the composition,        a reinforcing inorganic filler such as silica, in reduced quantities, by way of reinforcing filler instead of carbon black (which is excluded in these translucent compositions).        
In the present application, “reinforcing inorganic filler”, in known manner, is understood to mean an inorganic or mineral filler, whatever its color and its origin (natural or synthetic), also referred to as “white” filler or sometimes “clear” filler in contrast to carbon black, this inorganic filler being capable, on its own, without any other means than an intermediate coupling agent, of reinforcing a rubber composition intended for the manufacture of tires, in other words being capable of replacing a conventional tire-grade carbon black filler in its reinforcement function.
Advantageously, the entirety or at the very least a majority proportion of said reinforcing inorganic filler is silica. The silica used may be any reinforcing silica known to persons skilled in the art, in particular any precipitated or fumed silica having a BET surface area and a CTAB specific surface area both of which are less than 450 m2/g, even if highly dispersible precipitated silicas are preferred. Silica having BET or CTAB specific surfaces in the range of 80 m2/g to 260 m2/g are preferably used.
In the present specification, the BET specific surface area is determined in known manner, in accordance with the method of Brunauer, Emmet and Teller described in “The Journal of the American Chemical Society”, vol. 60, page 309, February 1938, and corresponding to AFNOR-NFT-45007 (November 1987); the CTAB specific surface area is the external surface area determined in accordance with the same AFNOR-NFT-45007 of 1987.
“Highly dispersible silica” is understood to mean any silica having a very substantial ability to disagglomerate and to disperse in an elastomeric matrix, which can be observed in known manner by electron or optical microscopy on thin sections. As non-limitative examples of such preferred highly dispersible silicas, mention may be made of the silicas Ultrasil 7000 and Ultrasil 7005 from Degussa, the silicas Zeosil 1165MP, 1135MP and 1115MP from Rhodia, the silica Hi-Sil EZ150G from PPG, the silicas Zeopol 8715, 8745 and 8755 from Huber, and treated precipitated silicas such as, for example, the silicas “doped” with aluminum described in application EP-A-735 088.
The physical state of the reinforcing inorganic filler is immaterial, whether it be in the form of a powder, microbeads, granules or alternatively balls. Of course, “reinforcing inorganic filler” is also understood to mean mixtures of different reinforcing inorganic fillers, in particular of highly dispersible silicas as described above.
As reinforcing inorganic filler, there may also be used for example:                aluminas (of formula Al2O3), such as the high-dispersibility aluminas which are described in the European patent document EP-A-810 258, or alternatively,        aluminum hydroxides, such as those described in the international patent document WO-A-99/28376.        
Also suitable are reinforcing inorganic fillers comprising carbon blacks modified by silica such as, for example, the fillers sold by CABOT under the name “CRX 2000”, which are described in the international patent document WO-A-96/37547.
The translucent rubber composition according to the invention furthermore comprises, in conventional manner, a reinforcing inorganic filler/elastomeric matrix bonding agent (also referred to as coupling agent), the function of which is to ensure sufficient chemical and/or physical bonding (or coupling) between said inorganic filler and the matrix, while facilitating the dispersion of this inorganic filler within the matrix.
Advantageously, the or each transmission means based on this elastomer or elastomers can have an axial cross-section with a relatively large area, because of the fairly insignificant difference in rigidity compared with the adjacent rubber compositions, and can on the other hand have a relatively small coefficient of transmission for a given quantity of energy transmitted.
According to a preferential embodiment of the invention in accordance with this second example, the tire is such that all the rubber compositions situated radially inside the wear indicator or indicators (i.e. including the or each transmission means) are translucent, comprising neither carbon black nor darkening agent.
According to a third example embodiment of the invention, the or each transmission means can advantageously comprise a core intended to transmit radiation, based on at least one relatively rigid material, as defined above in the first example embodiment, which is enveloped with a compressible deformable sheath designed to mechanically decouple the core from the adjacent rubber compositions in the tire. This sheath is preferably based on a cellular rubber and has a modulus of elasticity MA10, measured in accordance with ASTM D 412 of 1998, of between 0.1 MPa and 1 MPa.
This transmission means comprising the core and the sheath has for example the form of a bar whose side surface (consisting of the sheath) can advantageously be cylindrical, prismatic, conical or pyramidal in shape.
It should be noted that this core can advantageously have both a high coefficient of transmission and an also large axial section area, like the second example embodiment.
The sheath is preferably based on a cellular rubber with closed cells in order to prevent the absorption of water and in particular the diffusion of inflation gas in the structure of the tire (in particular in the plies and cables of the carcass reinforcement).
By way of example, it is possible to use glass for the core and, for the sheath, butyl foam, polychloroprene, a butadiene and acrylonitrile copolymer (NBR), an ethylene, propylene and diene terpolymer (EPDM), or a rubber composition used in the tire which has been expanded.
The tires according to the invention can be manufactured as follows.
According to a first embodiment of the invention, each radiation transmission means is formed in the tire during manufacture, before the curing of the corresponding cover (“cover” means in the present invention the uncured tire, sometimes referred to as the “blank”). It is in this case necessary for the material or materials making up each transmission means to be able to withstand the curing temperature of the tire (typically around 170° C.) without damage.
According to a second preferred embodiment of the invention, each transmission means is formed in the tire after the operation of curing the corresponding cover, preferably by the insertion and possibly adhesive bonding of the or each transmission means in a recess formed in the cured tire.
In accordance with this last preferred embodiment, the or each transmission means has substantially the shape of a drawing pin comprising a head provided with a stem at its center, the stem being contained in a radial recess formed from the internal face to the top of the corresponding tread pattern element, so that the head is in contact with the internal face and the free end of the stem is practically flush with the surface of the top, the head and part of the stem extending from the head being formed from the translucent rubber composition whilst the other part of the stem contained in the free end is formed from at least one opaque rubber composition, the interface radial height between the translucent composition and the opaque composition being designed to correspond to a wear threshold to be detected in the tread pattern element.
According to one advantageous embodiment of the invention, the tire comprises several radiation transmission means which have respectively different colors in the spectrum of the visible range.
Advantageously, the or each transmission means has a cross-section in the axial direction of the tire whose area increases continuously over a given wear height radially towards the inside of the tire, so that the quantity of energy of the radiation transmitted in the internal space increases continuously with the area of the section flush with the surface of the top of the element, when the latter wears.
According to one advantageous embodiment of the invention, the or each transmission means extends over the entire circumference of the tire.
Advantageously, the tire can comprise a plurality of transmission means whose respective heights, in the radial direction of the tire, correspond to various predetermined wear thresholds to be detected for the element.
In the aforementioned case in which the or each radiation transmission means comprises an optical fiber opening out in the internal space, the tire can be provided with at least one wear detection unit which contains:                in a radially uppermost upstream part passing through the internal face and ending in one of the tread pattern elements recessed from its top, the transmission means, which comprise one or more optical fibers parallel to each other which are each designed so as to be able to transmit incident radiation issuing from the space external to the tire to the space internal thereto and whose radial height or heights correspond respectively to one or various predetermined wear thresholds to be detected for the element, and        in a radially lower downstream part mounted in the space internal to the tire, a detection means connected to the optical fiber or fibers which is designed to detect the energy of the radiation transmitted to it by the fiber or fibers, from the time when at least one of the fibers is flush with the surface of the top of the tread pattern element.        
According to another characteristic of the invention, the wear detection unit comprises, in the downstream part, a transmitter coupled to an antenna for transmitting electrical signals representing measurements of radiated energy and a microprocessor for processing the radiated energy measurements.
Another aim of the present invention is to propose a wheel comprising a rim intended to receive a tire as defined previously, for implementing the also aforementioned wear measurement method.
According to the invention, the wheel rim is provided with a means of detecting the energy of the radiation transmitted by the or each transmission means in the space internal to the tire, as from the time when the or each transmission means are flush with the surface of the top of the tread pattern element because of wear on the latter.
Advantageously, the wheel rim is also provided with a means of quantifying the energy of the radiation transmitted in the space internal to the tire and/or means of locating in the tire each radiation transmission means.
Also advantageously, the wheel rim is also provided with a means of discriminating the colors of the visible radiation transmitted.
According to a preferential embodiment of the invention, the detection means is contained in a wheel module which is mounted on the wheel rim and which is intended to monitor the operating parameters of the tire/wheel assembly, such as its internal pressure and temperature, the wheel module also comprising a transmitter coupled to an antenna for transmitting electrical signals representing measurements of radiated energy, temperature and pressure, and a microprocessor intended to process all the measurements of radiated energy, temperature and pressure.
According to one advantageous example embodiment of the invention, the wheel rim is also provided with means for picking up the radiation received at the surface of the rim in radial directions with respect to the rim surface, and for concentrating this radiation picked up by directing it practically along the rim surface to the radiation detector.
To this end, the rim is for example provided at its bottom with a collar for holding the wheel module, the radially uppermost face of the collar comprising the means for picking up and concentrating the radiation.
Another aim of the present invention is to propose a tire/wheel assembly for implementing the aforementioned wear measurement method, comprising a wheel and a tire mounted on the wheel.
According to the invention, the tire/wheel assembly is such that the tire and the wheel are as defined previously.
Another aim of the invention is to propose a motor vehicle which comprises at least one tire/wheel assembly according to the invention, this vehicle being provided with a receiver intended to receive the signals from the transmitter, a computer connected to the receiver which is intended to process these signals and a display installed in the vehicle cabin which is connected to the computer and which is intended to inform the vehicle driver of the state of wear of the tread pattern elements on the tire or tires.